In the field of gasoline engines, so-called lean burn engines have been developed in order to reduce fuel consumption, which are fueled with lean air/fuel mixtures when operating under partial load. A lean air/fuel mixture contains a higher concentration of oxygen than necessary for complete combustion of the fuel. The corresponding exhaust gases then contain an excess of the oxidizing components oxygen (O2), nitrogen oxides (NOx) compared to the reducing exhaust gas components carbon monoxide (CO), hydrogen (H2) and hydrocarbons (HC). Lean exhaust gas usually contains 3 to 15 vol.-% oxygen. However, during operation under load and full load, stoichiometric or even substoichiometric, i.e. rich, air/fuel mixtures are used even in lean burn engines.
Diesel engines on the other hand usually run under operating conditions with highly superstoichiometric air/fuel mixtures. Only in recent years have diesel engines been developed that can also be operated with rich air/fuel mixtures for short periods of time. In the present invention, diesel engines, in particular those with possible rich operating phases, are referred to as lean burn engines as well.
Due to the high oxygen content of the exhaust gases from lean burn engines, the nitrogen oxides contained therein cannot be continuously reduced to nitrogen in combination with a simultaneous oxidation of hydrocarbons and carbon monoxide by means of so-called three-way catalysts as is the case in stoichiometrically operated gasoline engines. Rather, with these catalysts a so-called temperature window for the reduction of the nitrogen oxides, which depends on the exhaust gas temperature, is observed. An increase in the exhaust gas temperature results in an initial increase in the nitrogen oxides conversion. At a certain temperature the conversion rate reaches a maximum and at higher temperatures the conversion rate recedes back to zero. Within the temperature window, the remaining hydrocarbons that are always still present in lean exhaust gas function as reducing agents for the nitrogen oxides.
The position and width of the temperature window as well as the maximum nitrogen oxide conversion within the temperature window depend on the formulation of the catalyst and the residual hydrocarbon content of the exhaust gas. Conventional three-way catalysts only show a low nitrogen oxides conversion within the temperature window. However, so-called HC—DeNOx catalysts were developed, which exhibit a maximum nitrogen oxides conversion in the temperature window of up to 60% at a temperature in the range of 180 to 250° C. The width of the temperature window is only about 50° C.
Despite the relatively high nitrogen oxide conversion rate within the temperature window, these catalysts only provide an average nitrogen oxide conversion of less than 30% throughout the standardized driving cycle MVEG-A.
In order to improve this situation, so-called nitrogen oxides storage catalysts were developed which store the nitrogen oxides contained in lean exhaust gas in the form of nitrates.
The mechanism of nitrogen oxides storage catalysts is described in detail in the SAE document, SAE 950809. Accordingly, nitrogen oxides storage catalysts consist of a catalyst material that commonly is applied on an inert, ceramic or metal honeycomb carrier, a so-called carrier, in the form of a coating. The catalyst material comprises the nitrogen oxides storage material and a catalytically active component. The nitrogen oxides storage material in turn consists of the actual nitrogen oxides storage component, deposited in highly dispersed form on a support material.
Basic alkali metal oxides, alkaline earth metal oxides and rare earth metal oxides, and in particular barium oxide, which react with nitrogen dioxide to form the corresponding nitrates, are predominantly used as storage components. It is known that in air these materials are mostly present in the form of carbonates and hydroxides. These compounds are also suitable for storing the nitrogen oxides. Thus, whenever basic storage oxides are mentioned in the present invention, this also includes the corresponding carbonates and hydroxides.
Noble metals of the platinum group are typically used as catalytically active components, which as a rule are deposited on the support material together with the storage component. Active aluminum oxide with a large surface area is usually used as support material. However, the catalytically active components can also be applied on a separate support material such as for example active aluminum oxide.
It is the task of the catalytically active components to convert carbon monoxide and hydrocarbons to carbon dioxide and water in the lean exhaust gas. Furthermore, they should oxidize the nitrogen monoxide portion of the exhaust gas to nitrogen dioxide so that it can then react with the basic storage material to form nitrates (storage phase). An increasing incorporation of the nitrogen oxides in the storage material causes a decrease in the material's storage capacity, which has to be regenerated from time to time. For this purpose, the engine is operated for a short period of time with stoichiometric or rich air/fuel mixtures (referred to as regeneration phase). In the reducing conditions of the rich exhaust gas, the formed nitrates decompose to nitrogen oxides NOx and, with the use of carbon monoxide, hydrogen and hydrocarbons as reducing agents, are reduced to nitrogen while water and carbon dioxide are formed.
During the operation of the nitrogen oxides storage catalyst, the storage phase and the regeneration phase regularly alternate. Usually, the storage phase lasts between 60 and 120 seconds, while the regeneration phase is completed in less than 20 seconds.
Nitrogen oxide storage catalysts allow considerably higher nitrogen oxides conversion rates in a larger temperature window than HC—DeNOx catalysts. Their nitrogen oxides conversions meet the exhaust limits according to the Euro IV standard, the introduction of which is planned for 2005.
However, in order to improve the safety of operation and long-term stability of these catalysts, it is necessary to increase their thermal stability, widen their temperature window and further improve the nitrogen oxides conversions attainable in that window.
Based on the forgoing, there is a need in the art for a catalyst for the prevention of nitrogen oxides in the exhaust gas from combustion engines, which has an improved thermal stability, a wider temperature window and a higher nitrogen oxides conversion rate in this window than conventional nitrogen oxides storage catalysts.